Method for electrolytically shaping group 5b metals



United States Patent 3,245,891 METHOD FOR ELECTROLYTICALLY SHAPING GROUP53 METALS Simon P. Gary, Villa Park, Ill., assignor to AnocutEngineering Company, Chicago, 11]., a corporation of Illinois N0Drawing. Filed Mar. 23, 1962, Ser. No. 182,118 8 Claims. (Cl. 204-143)This invention relates to an electrolyte for electrolytic shaping of ametallic workpiece.

The art of electrolytic shaping has been well developed for a variety ofmetals and alloys, but niobium and tantalum and alloys containingsubstantial proportions of these metals have been substantiallyimpervious to such shaping treatment.

The process, as commonly practiced, consists of placing an electrodenear a metallic workpiece and flowing an electrolytic solution rapidlythrough the gap between. The gap may range from .0002" to .02". Thepressure driving the electrolyte through the gap will range from 30 to300 pounds per square inch. The electrode is connected to constitute thecathode and the workpiece, the anode. An electrolyzing current isemployed having a density of 500 to 8000 amperes per square inch at to24 volts. The penetration rate is roughly .10 a minute per 1000 amperesper square inch current density.

The advantages of electrolytic shaping are likewise well known. Metalmay be removed at a high rate of speed and leave a smooth and polishedsurface. The rapidity of metal removal is not dependent on the hardnessof the material but rather on its chemical activity. It can formexceedingly complex shapes inconceivable under conventional machiningmethods. The application of this method, however, to the metals of group5B of the periodic table has not hitherto been successful.

This inventon has as a major object a method for electrolyticallyshaping these metals and alloys containing them in suflicient quantityto be resistive to conventional electrolytic treatment, and theprovision of an electrolyte whereby such shaping becomes possible.

The invention further includes as a major object the provision of anelectrolyte suitable for such shaping which is relatively inexpensive.

The invention resides in the provision of an electrolytic material whichwill form bromide ions at the work piece anode. Material is removed inthe form of niobium or tantalum bromides and is carried away by thestream of electrolyte from the working area.

A suggested source of the bromide ions is sodium bromide or potassiumbromide, depending on which is the less expensive although calciumbromide or ammonium bromide will serve as well. The requirements of thematerial contributing the bromide ion are that it be highly soluble inwater, that its cost be low, that it not hydrolize, and that thepositive ion not plate out in the course of the shaping. Any ion morepositive than uranium on the electromotive scale is suitable. The normaloperating temperature of the electrolyte in the shaping of thesematerials is in the vicinity of 100 to 120 F. The temperature should beas high as possible consonant with an absence of cavitation or bubbleproduction in the vicinity of the electric flow. (Some bubble formationis inevitable in that hydrogen is evolved from the anodic workpiece, butthe rate of electrolyte flow should be sufiicient to sweep such bubblesaway before they interefere significantly with the process.) This rangeof temperatures, however, successfully avoids such formation. On theother hand, this temperature is naturally maintained by the resistanceheating thereof from the high current flow and, to a minor degree, bythe frictional heating of the electrolyte in its passage through theworking area.

The concentration of the bromide may vary widely. The concentration, ofcourse, will affect both the quantity of bromide ions formed and theconductivity of the solution. A saturated solution of sodium bromidewill permit an electrode penetration of the workpiece of about .05" perminute at a current density of 1000 amperes per square inch. This ratewill remain substantially unchanged down to a concentration of about twoounces of sodium bromide per gallon. The rate will then dwindleprogressively more rapidly as the concentration diminishes to a pointwhere, with about one-tenth of an ounce of sodium bromide per gallon,the rate of removal is negligible.

A problem to be considered in the electrolytic shaping of these metalsresides in the relatively high cost of sodium bromide as compared withconventionally employed electrolytic salts, notably sodium chloride. Thebromides of the group 5B metals are unstable in aqueous solution anddecompose into the hydroxides (or equivalent acids), but thedecomposition takes place relatively slowly. Exhaustion of the freebromide ion, or at least a material reduction thereof, in the course ofthe chemical combination incident to the shaping must be considered. Afurther consequence of using a low concentration of sodium bromide issimply that desirable current densities under desirable voltages forthis sort of operation cannot be obtained.

To meet these difficulties, it is proposed that a substantial proportionof sodium chloride or similar salt be incorporated in the electrolyticsolution along with the bromide salt. The sodium chloride provides manyadvantages. It permits the use of a relatively low concentration of themore expensive bromide and at the same time keeps the conductivity ofthe electrolyte high. It provide a solution having high conductivitywhich is inexpensive enough either to avoid the necessity ofrecirculating the electrolyte or at least permit a great volume ofelectrolyte with reference to the flow rate of the electrolyte throughthe reaction zone so as to provide for a substantial period of time inwhich the metallic bromides formed may decompose and precipitate out toreconstitute the sodium bromide. The avoidance of recirculationgenerally is to be desired, or at least recirculation within a shortperiod of time, not only because of the temporary exhaustion of thebromide ions by the reaction but also because of the possibility ofplating out the group 5B metals on parts of the workpiece away from thereaction zone.

Although the invention is operative over a wide range of bromideconcentration with or without an admixture of sodium chloride, the threefollowing specific formulations will illustrate the nature of theelectrolyte.

Example A Water gal 1 Sodium bromide (.07 N) oz 1 Example A isillustrative of an electrolyte using sodium bromide only. The sodiumbromide may range from one quarter of an ounce to four ounces per gallonof water (.018 to .29 N). There is no advantage in going to a higherproportion of sodium bromide than four ounces and the effectiveness ofthe electrolyte falls off very fast below a quarter of an ounce pergallon. The rate of removal at a quarter of an ounce per gallon isslower than with the stated one ounce, on the order of one-half to threequarters of the rate, but may still be suitable for some purposes. At aquarter ounce, the concentration is near an exhaustible level of bromideions.

Example B Water gal 1 Sodium bromide (.007 N) oz Sodium chloride oz 2Example B illustrates the use of minimum quantities of sodium bromidetogether with a proportion of sodium chloride to provide sufiicientconductivity for the electrolytic shaping to take place. The proportionsin Example B are minimal. With a reduction of either of the salts, theeffectiveness of the solution falls off very rapidly. Again, the bromideconcentration lies just above the point where bromide ion exhaustion islikely to appear. The penetration rate in Example B is about one-quarterto one-third of that possible with Example C.

Example C Water gal 1 Sodium bromide (.14 N) oz 2 Sodium chloride lb 1simply more conductive ions and from the aspect that a sodium chloridesolution is a better conductor than a Y sodium bromide solution. ExampleC recites a minimumcost, maximum-effectiveness proportion. In thisproportion, a penetration rate has been obtained of .05" per minute at1000 amperes per square inch or .10" at 2000 amperes per square inch. Areduction of sodium bromide will slow the rate of penetration. A greateramount will gain nothing other than added cost. The sodium chloridecontributes the necessary conductivity for maximum penetration.

It will be appreciated from the foregoing description that anelectrolyte has been devised which makes possible the electrolyticshaping of the group 53 elements of the periodic table, and moreparticularly niobium and tantalum, which is inexpensive and effective.It will be appreciated that any of a number of bromide salts may beemployed, such as potassium bromide, calcium bromide or ammoniumbromide. The requirements are that the salt be soluble so as tocontribute enough bromide ions, that the metal ion of the salt not plateout and that it be cheap.

The sodium chloride is present, as stated, solely for purposes ofconductivity. Any salt contributing like conductivity and free from sideeffects or reactions under the circumstances of the shaping will serveas well. The one distinguishing characteristic of sodium chloride is itscost.

Of the group 5B elements, niobium and tantalum have been discussedprimarily because they are playing an increasing part, in relativelypure form, in high temperature technology. The third member of thegroup, vanadium,

is likewise relatively inert to electrolytic attack with a chlorideelectrolyte, and the presence of the bromide ion makes practicableelectrolytic shaping of it to the same degree as the other two elements.

Although the concentration of the bromide has been stated generally interms of ounces per gallon, obviously the significant factor is the ionconcentration. The electrolyte, therefore, is defined in some of theclaims herein in terms of a fraction of normal solution.

This invention, therefore, should be regarded as being limited only asset forth in the following claims.

I claim:

1. A method for the electrolytic shaping of a workpiece of the group 5Belements of the periodic table and their base alloys which comprisesflowing an essentially purely aqueous electrolyte between an electrodeand such workpiece which are in close proximity to each other,-.theelectrolyte having a concentration of at least .007 normal of anionizing, non-hydrolyzing bromide of a positive ion above uranium on theelectromotive scale, and passing an electrolyzing high density currentbetween the electrode and the workpiece in a sense to make the workpieceanodic.

2. The method as set forth in claim 1 wherein the bromide salt is takenfrom the group consisting of sodium, potassium, and ammonium bromides.

3. The method as set forth in claim 1 wherein the electrolyte is atleast .018 normal.

4. The method as set forth in claim 1 wherein there is an ionizing saltin the electrolyte which is stable under the condition of theelectrolysis and in the presence of the bromide salt, in sufficientconcentration to impart to the electrolyte a conductivity to permit acurrent density of about 1000 amperes per square inch at about six voltsbetween the electrode and the workpiece.

5. The method as set forth in claim 1 wherein the electrolyte is from.018 to .3 normal.

6. The method as set forth in claim 1 wherein the electrolyte containsfrom one-fourth to four ounces sodium bromide per gallon of water.

7. The method as set forth in claim 1 wherein the electrolyte includes,per gallon of water, from one-tenth to two ounces of a bromide takenfrom the group consisting of sodium, potassium, and ammonium bromidesand from two ounces to one pound of a salt having generally the ionizingcharacteristics and the stability under the electrolysisand in thepresence of the bromide salt of sodium chloride.

8. The method as set forth in claim 1 wherein the bromide salt iscalcium bromide.

References Cited by the Examiner UNITED STATES PATENTS 2,742,416 4/1956Jenny 204-141 2,863,811 12/1958 Ruscetta 204-141 2,920,026 1/1960Kistler 204 143 3,130,138 4/1964 Faustetal 204 143 FOREIGN PATENTS1,241,349 8/1960 France.

JOHN H. MACK, Primary Examiner.

JOSEPH REBOLD, Examiner.

1. A METHOD FOR THE ELECTROLYTIC SHAPING OF A WORKPIECE OF THE GROUP 5BELEMENTS OF THE PERIODIC TABLE AND THEIR BASE ALLOYS WHICH COMPRISESFLOWING AN ESSENTIALLY PURELY AQUEOUS ELECTROLYTE BETWEEN AN ELECTRODEAND SUCH WORKPIECE WHICH ARE IN CLOSE PROXIMITY TO EACH OTHER, THEELECTROLYTE HAVING A CONCENTRATION OF AT LEAST .007 NORMAL OF ANIONIZING, NON-HYDROLYZING BROMIDE OF A POSITIVE ION ABOVE URANIUM ON THEELECTROMOTIVE SCALE, AND PASSING AN ELECTROLYZING HIGH DENSITY CURRENTBETWEEN THE ELECTRODE AND THE WORKPIECE IN A SENSE TO MAKE THE WORKPIECEANODIC.